Efforts have been made to improve the selectivity of the sulfurization of phenols and reduction thereof to mercaptophenols. The sulfurization of phenols has heretofore produced large quantities of impurities such as monosulfides, which are not reducible to mercaptophenols. This lack of process selectivity has also prevented the isolation of high purity mercaptophenols due to the similar physical profiles of the mercaptophenols and side-produced impurities. Known sulfurization processes employing sulfur chloride for the production of phenolic sulfides in which phenolic rings are linked by one or more sulfur atoms continue to be unsatisfactory sources for mercaptophenols. The difficulty is that the action of sulfur chloride on phenols results in mutation reactions and sulfur deposits which constitute mixtures of mono- and poly-sulfides wherein the amount of monosulfide prevails.
Heretofore, efforts to improve the selectivity of the sulfurization of phenols with sulfur chloride and the subsequent reduction to mercaptophenols have been directed to the use of sterically-hindered phenols, and/or variations of catalysts, for example, metal or metal-containing catalysts, solvents, additives, and the like. These efforts not only fail to report on the effect of reducing the concentration of phenols in a reaction admixture, but, in fact, the reported works would direct a conclusion that decreasing the concentration of phenols should produce a slight to marked decrease in yield of polysulfides of phenols which are reducible to mercaptophenols. Furthermore, the use of metal-containing catalysts adds an undesirable component to the reaction system. The additive metal ultimately appears as an unwanted impurity in the product formed thereby requiring additional removal steps to eliminate it from the system. This removal operation is both time-consuming and costly.
I have found that a process having good conversion of phenols coupled with high selectivity for polysulfides, therein providing a desirable source for mercaptophenols, results from a reaction slurry system containing low concentrations of phenols and sulfur chloride, in a polar solvent reaction medium, the slurry including a catalytic amount of hydrogen chloride promoter. Quite surprisingly, I have found that by reducing the concentration of phenols from about 1% to about 25% by weight of the reaction admixture, improved selectivity for polysulfides of phenols reducible to the corresponding mercaptophenols is achieved. However, low concentrations of phenols can cause slower reaction rates. Therefore, a catalytic quantity of a hydrogen chloride promoter is employed in the subject reaction system to increase the rate of selective sulfurization of phenols without indroducing impurities to the system caused by the addition of metals or metal-containing catalysts. The reaction admixture containing the reducible polysulfides can be reduced by known methods, for example, the hydrogenating-splitting of polysulfides to mercaptans. The mercaptophenols produced according to this process can be isolated in a relatively high purity form as a direct result of the selectivity of the sulfurization reaction.
Therefore, it is the primary objective of this invention to provide a selective process for the production of mercaptophenols from the sulfur chloride sulfurization of phenols in a reaction slurry containing a polar solvent and a catalytic amount of hydrogen chloride, and the subsequent reduction of the resulting polysulfides. Another objective of this invention is the production and isolation of high purity mercaptophenols by the above described process which controls undesired mutation reactions and sulfur deposits which can result from the uncontroled action of sulfur chloride on phenols.